Only use heat image where support

Glance will reject images that it doesn't understand, and only certain drivers can
use qcow2 images.  This means for systems without qcow2 support, devstack
will fail to run until this change is merged.

Change-Id: Ic92815f3d32880e987456c0428953dddaa81c94c
Merge branch 'master' of github.com:openstack-dev/devstack